If you already have a router and you are adding a wifi access point, you may find that your wifi device tries really really hard to also be the router.
Here are some steps to make it STFU and provide wifi access:
- Reset to factory default.
- Connect a computer to it with a cable using one of the LAN ports.
- Log in to it.
- Change its IP address to one that is in the main router’s subnet but outside its DHCP scope. Disable DHCP.
- Reboot it.
- Connect your network via a LAN port rather than the internet port.
These steps copied and tweaked from here